1. Field of the Invention
This invention relates to the field of cylindrical film-type resistors.
2. Description of Prior Art
The present invention constitutes an improvement over what is shown and described in U.S. Pat. No. 3,858,147, inventor Richard E. Caddock. Such patent is hereby incorporated by reference herein, as though set forth in full.
The cited patent shows and describes a silk-screened serpentine resistive film pattern which is uniform in width. Stated otherwise, the serpentine "line" of resistive material is shown as having a constant width throughout the full length of the line, the bend regions of the line being no wider than are the parallel arm regions thereof. The cited patent also specifically discloses two ways of trimming the resistive film in order to achieve the desired ohmic value. One such way is to abrade a predetermined small region of the film, and a second such way is to partially grind the entire film exterior.
The first-mentioned method of trimming is relatively uneconomic and/or disadvantageous. For example, when the predetermined small abraded region extends circumferentially of the cylindrical substrate--which is believed to be usually the only practical thing to do relative to the present serpentine film pattern--there may be created a circumferentially-extending weak spot (stress-riser line) in the substrate. Such circumferential stress riser increases the chances that the substrate will break in two when stressed.
The second-mentioned method of trimming, namely grinding (or lapping) the entire exterior of the film, also produces disadvantages particularly relative to film materials having very high resistances. It is pointed out that when a film-forming resistive material has a high inherent resistance, there will be relatively few conductive paths (microscopic in size) through any predetermined small region thereof. Conversely, a film-forming material having a low inherent resistance will be characterized by the presence of many conductive paths through any such predetermined region. Thus, when a high-resistance film is partially abraded or lapped away, to reduce its thickness and thereby increase its resistance, there is a distinct chance of creating a discontinuity or interruption in the line of film since all or substantially all of the microscopic flow paths may be broken.
It is strongly emphasized that the discontinuity or interruption described in the preceding paragraph may not manifest itself immediately, but instead only after passage of time and changes in environment have occurred. Thus, in the absence of the present invention, instabilities may develop in the field.
Relative to prior art other than the cited Caddock patent, it is known to provide serpentine resistive films in which the bend regions are relatively wide in comparison to the remaining regions. However, there is no prior art known to applicant wherein a wide-bend pattern is employed in conjunction with a longitudinal gap in the film, or with lapping (or grinding) of the entire exterior of the film. For these and other reasons, the present article and method are not disclosed or suggested by the prior art.